The invention relates to relays which switch AC power to a load. More particularly, the invention relates to a relay driver circuit which prevents relay contact degradation.
The potential across and current flowing between the contacts of a relay when they open and close cause the migration of melted or ionized contact material from one contact to the other. The direction of material migration depends upon the polarity established across the relay contacts at the time when they open or close. If the polarity across the contacts at the instant they open or close is consistently in the same direction, contact degradation results since one of the contacts repetitively gives up material while the other contact repetitively accumulates material. The relay can eventually malfunction due to contact pitting which causes mechanical welding.
The above-described degradation of a relay occurs in several applications. One such application involves air conditioning systems wherein a timer delay circuit is used to operate a blower fan while the compressor is on and to keep it operating for a pre-determined period of time following compressor shut-off. The operation of the blower fan after compressor shut-off allows higher energy efficiency of the air conditioning system.
A thermostat controls the operation of the air conditioning system. When the thermstat switches to its "on" mode in response to a temperature rise, an AC signal, typically 24 volts, is switched to actuate the compressor. This AC signal is also switched to a trigger input of the delay timer. The delay timer is triggered on a positive cycle of the AC signal when the voltage is above a certain threshold. When the delay timer is triggered, its output turns on and causes the energizing of a relay, which correspondingly closes an AC circuit consisting of the blower fan.
When the temperature lowers and causes the thermostat to turn off, the AC signal is not provided to actuate the compressor or to trigger the delay timer. As a result, the compressor shuts off, while the fan continues to operate until the predetermined time delay of the timer has elapsed. When the time has elapsed, typically 80 seconds in this application, the output of the delay timer shuts off which deenergizes the relay coil. Consequently, the AC circuit providing power to the fan is turned off.
The AC signal switched by the thermostat and provided to the delay timer is synchronized in frequency with the AC source operating the fan. The switching of the thermostat is independent of the phase of the AC signal. However, since the delay timer is triggered upon only a positive cycle of the AC signal, the relay controlling operation of the fan closes its contacts repetitively on the same portion of a cycle of the AC source. The closing of the relay contacts consistently on a positive cycle or on a negative cycle results in contact degradation.
As a result, an inexpensive method to alleviate the contact degradation occurring in this application is desired. Methods have been devised which protect a relay from contact degradation. In one method, a relay is actuated when a control signal reaches a certain DC threshold. However, since the control signal also consists of an AC line voltage component, the threshold voltage is reached repetitively during the positive cycles of the AC line voltage. Contact degradation occurs as a result. To prevent the relay contacts from closing repetitively on a positive cycle of the line voltage, a separate signal generating circuit is used to generate a timing signal which has a cycle longer than, and being asynchronous with, the AC line voltage. In order to close the relay, a specified combination of both the generated timing signal and the control signal must be present. This dependence on the asynchronous timing signal consequently results in the relay to close at random times with respect to the phase of the AC line voltage.
However, applying this method to alleviate the problem associated with the air conditioning system would be overly expensive and complex. The present invention provides an inexpensive method for protecting a relay from contact degradation when applied to systems similar to that of the air conditioning system.